API REPORT 93-77:2002

Cathodically protected welded joints for marine service are traditionally designed (1,2) using S-N curves derived from tests of similar joints in air. This approach is based on the assumption that adequate cathodic protection restores the fatigue life in seawater to values measured in air (3). However, extensive testing of welded plate and tubular joints during the past fifteen years has revealed that, at least at short lives and low cyclic frequencies, the corrosion fatigue lives of cathodically protected joints are significantly shorter than those in air. Most corrosion fatigue data became available from major national and international research programs: The United Kingdom Offshore Steels Research Project (UKOSRP), the European Coal and Steel Community (ECSC) sponsored program, the Canadian and Norwegian national programs, Japanese projects, the UK Cohesive Research Programs supported by the U.K. Science and Engineering Research Council (SERC), and more recently from American (NI sponsored), and Chinese projects. Data on the effects of cathodic protection were reviewed in 1986 (4). That review concluded that the fatigue life of cathodically protected welded joints in seawater is shorter than in air by about a factor of two. Subsequently the basic design S-N curve for protected joints in the Canadian code for offshore structures was reduced by a factor of two (5). The accumulated new experimental data prompted a recent major revision of the UK offshore guidance (2). In the proposed revision (6,7), a penalty factor of two is applied on the air S-N curve, to obtain the design curve for cathodically protected joints at short lives. At long lives (N 10(7)), cathodic protection is assumed to mitigate the adverse effect of seawater, and the section of the air S-N curve with increased slope (m = 5) becomes applicable. For intermediate lives, the long life section of the curve is extrapolated backwards to an intersection with the short life section. The Canadian and UK approaches to design are in agreement for short lives. The paucity of relevant corrosion fatigue data at long lives (required test times are longer than one year), makes the evaluation of cathodic protection effects uncertain. However, the CSA code (3) is based on data reviewed in 1986. The data for the UK proposed revision (6,7) were reviewed in 1990. Since then more research has been completed. The objective of the present review of the upgraded database is to place the evaluation of the long term effects of cathodic protection on the soundest possible technical base. A selective database has been used in this review. Only results from plate and tubular joints welded to offshore standards and tested under conditions simulating the offshore environment are included. Tests of plate joints are used to evaluate seawater effects on fatigue life under conditions of optimum cathodic protection (OCP), cathodic overprotection (COP), and free corrosion (FC). The few corrosion fatigue tests of tubular joints are used to verify the environmental effects measured on plate joints.